It was a cooperative effort between federal agents and city police. And the result was one they all hoped and planned for: a safe event.

"The planning and the preparation, all of the coordination has really led to a very successful race in terms of race operations," Mayor Nutter said nearly three hours after the starting gun fired, when the vast majority of runners had crossed the finish line.

The April 15 twin bombings at the Boston Marathon had raised the anxieties of Broad Street runners and spectators, and also influenced the security plan for the day.

Hundreds of police officers were visible along the route, and the city had essentially prohibited backpacks at the event for the first time in its 34-year history. Participants were given clear plastic bags to hold their personal belongings.

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The two bombs at the Boston Marathon were in backpacks.

At the starting line in North Philadelphia, as Nutter danced to "Sweet Caroline" - regularly played at Boston Red Sox games - and showed off the red socks he wore in support, unseen security measures were taking place in addition to the visible display of law enforcement officers on bicycle and foot, and riding in cars.

Nutter would not talk about the hidden security measures, but said federal authorities helped in that area by being "extremely generous with their time, effort, and support."

The event, sponsored by Independence Blue Cross, had an estimated 38,000 runners; thousands more watched from the curb.

Detours were part of the security plan to protect runners. Fences were placed at the start and finish. Near the finish line at the I-95 overpass, spectators were directed to a back entrance into the Navy Yard, but runners were allowed to make a straight dash to the finish. Motorists on Old York Road on Sunday morning were rerouted up Tabor Road away from North Broad Street.

Nutter took note that almost every spectator complied with what was more a strong discouragement against the use of backpacks than an outright prohibition, saying: "Most people understood what was going on."

"We plan for what happened in Boston," SEPTA Transit Police Chief Thomas Nestel III said during the race. "We have the resources to handle that type of disaster. . . . We're trying to let people know they are safe."

Nearly two hours into the race, a visibly relieved Nutter said that no arrests had been made and that "no one has done anything they shouldn't do so far." The mayor did not run in the race but covered every mile of the course, using an escort vehicle to get him to various points.

Nutter said it was common for police to deploy helicopters from time to time but added, "Certainly for a day like today, we want to have eyes in the sky that allow us to have the visual from above to communicate on the ground."

He wouldn't comment on whether the Police Department would consider using drones, saying he would "deal with that issue another time."